Observing system experiments within the operational ECMWF data assimilation framework have been performed for summer 2008 when the largest recorded number of GNSS radio occultation observations from both operational and experimental satellites has been available. Constellations with 0, 5, 33, 67, and 100% data volume were assimilated to quantify the sensitivity of analysis and forecast quality to radio occultation data volume. These observations mostly constrain upper tropospheric and stratospheric temperatures and correct an apparent model bias that changes sign across the upper troposphere - lower stratosphere. This correction effect does not saturate with increasing data volume, even if more data is assimilated than available in today's analyses. Another important function of radio occultation data, namely the anchoring of variational radiance bias corrections, is demonstrated in this study. This effect also does not saturate with increasing data volume. In the stratosphere, the anchoring by radio occultation data is stronger than provided by radiosonde and aircraft observations.